The increasing density of integrated circuits (ICs)is driving the need for materials with high dielectric constants to be used in electrical devices such as capacitors for forming 256 Mbit and 1 Gbit DRAMs. Capacitors containing high-dielectric-constant materials, such as organometallic compounds, usually have much larger capacitance densities than standard SiO.sub.2 --Si.sub.3 N.sub.4 --SiO.sub.2 stack capacitors making them the materials of choice in IC fabrication.
One organometallic compound of increasing interest as a material for use in ultra large scale integrated (ULSI) DRAMs is BST due to its high capacitance. Deposition techniques used in the past to deposit BST include RF magnetron sputtering, laser ablation, sol-gel processing, and chemical vapor deposition (CVD) of metal organic materials.
A liquid source BST CVD process entails atomizing a compound, vaporizing the atomized compound, depositing the vaporized compound on a heated substrate and annealing the deposited film. This process requires control over the liquid precursors and gases from introduction from an ampoule into a liquid delivery system through vaporization and ultimately to the surface of the substrate where it is deposited. The goal is to achieve a repeatable process which deposits a film of uniform thickness under the effects of a controlled temperature and pressure environment. The goal has not been satisfactorily achieved because the precursors are finicky and the deposition equipment requires a complex design.
For example, one difficulty encountered is that the delivery of liquid precursors has typically required positive displacement pumps. Pumps can become clogged and require replacement if the precursors deposit on the surfaces of the pumping system. In addition, use of positive displacement pumps becomes problematic when the delivery lines or the vaporizer become clogged with deposits because the pump can rupture the pressure seals or continue to operate until the pressure relief valves on the pump are tripped. Either result may require maintenance and repair and over time repair and replacement of pumps becomes very expensive and increases the cost of ownership of the equipment.
Another difficulty encountered is that BST precursors have a narrow range of vaporization between decomposition at higher temperatures and condensation at lower temperatures thereby requiring temperature controlled flow paths from the vaporizer into the chamber and through the exhaust system. In addition, the liquid precursors tend to form deposits in the delivery lines and valves disposed throughout the system.
Another difficulty encountered is the difficulty or lack of efficiency in vaporizing the liquid precursors. Typically, only a portion of the liquid precursors are vaporized due to low conductance in the vaporizer, thereby inhibiting deposition rates and resulting in processes which are not consistently repeatable. In addition, known vaporizers used in CVD processes incorporate narrow passages which eventually become clogged during use and are not adapted for continuous flow processes which can be stabilized. This too results in a reduction in vaporization efficiency of the liquid precursors and negatively affects process repeatability and deposition rate. Still further, known vaporizers lack temperature controlled surfaces and the ability to maintain liquid precursors at a low temperature prior to injection into the vaporizer. This results in deposition of material in the injection lines in the vaporizer and premature condensation or unwanted decomposition of the precursors.
Still another difficulty encountered in the deposition of BST is that the deposition process is performed at elevated substrate temperatures, preferably in the range of about 400-750.degree. C. and the annealing process is performed at substrate temperatures in the range of about 550.degree.-850.degree. C. These high temperature requirements impose demands on the chambers used in the deposition process. For example, elastomeric O-rings are typically used to seal the deposition chamber and are not generally made of materials that will resist temperatures in excess of about 100.degree. C. for many fabrication cycles. Seal failure may result in loss of proper chamber pressure as well as contamination of the process chemistry and the system components, thereby resulting in defective film formation on the wafer. In addition, it is necessary to prevent temperature fluctuations of system components which result from thermal conduction. Loss of heat due to thermal conduction causes temperature gradients across the surface of the substrate resulting in decreased uniformity in film thickness and also increases the power demands required of the system to maintain the high temperature environment in the chamber.
There is a need, therefor, for a deposition apparatus and method which can deliver liquid precursors to a vaporizer, efficiently vaporize the precursors, deliver the vaporized precursors to the surface of a substrate and exhaust the system while maintaining elevated temperatures in the chamber, preventing unwanted condensation or decomposition of precursors along the pathway and avoiding temperature gradients in the system. It would be preferable if the system were adapted for rapid cleaning and continuous flow operation.